1. Field of the Invention
The invention relates to a process for producing an open cell foam of flexible polyvinyl chloride resin.
2. Description of Prior Art
Flexible polyvinyl chloride resin foams have previously been produced from general-purpose polyvinyl chloride, formed by suspension polymerization, in combination with a heat decomposable blowing agent such as azodicarbonamide. Such foams are of the closed cell type and it has been impossible, when using such a resin to obtain an open cell foam of the polyurethane foam type which exhibits substantial breathability and water permeability.
Open cell polyvinyl chloride foams can be obtained from plastisols produced from Polyvinyl chloride, formed by emulsion polymerization. The plastisol is used as a starting material in combination with a blowing agent particularly adapted for use with plastisols such as AZ-F (azodicarbonamide powder dispersed in paraffin) made by OTSUKA YAKUHIN K. K. as well as with a mixture of azodicarbonamide and vaseline.
While open cell foams can be obtained when using such plastisols in combination with specially selected blowing agents, open cell foams cannot be obtained when using azodicarbonamide alone. Additionally, blowing agents developed for use with plastisols cannot be applied to polyvinyl chloride produced by suspension polymerization for the following reasons:
1. It is very difficult to knead compounds containing such blowing agents.
2. Even if compounds containing such blowing agents could be kneaded, they could not be used to form a sheet because compound exhibits high lubricancy because of the presence of vaseline and paraffin.
3. Using such blowing agents insufficient gellation is achieved.
4. Dispersion of such blowing agents is so poor that uniform open cell foams cannot be obtained.
Furthermore, open cell foams produced by plastisols in "liquid phase coating techniques" including the doctor-blade process and the reverse roll coater process are poor in efficiency and high in material and operating costs as compared with foams which are produced by calendering.
There is, therefore, a demand for a process for producing open cell foams by calendering techniques. Yet, thus far no practical process for forming open cell foams by calendering is available.